cxgb3: Set vlan_feature on net_device

cxgb3 interface has a bad performance when VLAN is set. On my current
setup, a PowerLinux 7R2, I am able to get around 7 Gbps on a TCP_STREAM
(8 instances, 4k message).
With this patch, I am able to reach 9.5 Gbps.

Signed-off-by: Breno Leitao <brenohl@br.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>